What is the meaning of E 90 message

The E90 error code is not a specific code. The error code E90 is just a group error code. It is indicating a communication error between the two boards in your machine. It does not indicate where the problem may be. A power glitch may have caused the E90 error code or a loose connection. One of the control boards could have failed. If it did not clear by unplugging the washer" for 5 minutes. The first step to take is check and clean all the wiring connections at both boards, look for loose wires or bad solders, electrical contact cleaner will be a good source for cleaning any connections or pins on the boards or in the harness. If all checks are OK and clean, and the problem persists, then replace the "main" control board. Of course you need to unplug the machine before working on the wiring, also use a multimeter to check for continuity.